The present invention relates generally to medical imaging, and more particularly to a histogrammer for a medical imaging device such as a positron emission tomography scanner.
A positron emission tomography (PET) scanner detects gamma rays which emanate from the patient. In a PET scan, the patient is initially injected with a radiopharmaceutical, which is a radioactive substance such as FDG ([18F] fluorodeoxyglucose) which emits positrons as it decays. Once injected, the radiopharmaceutical becomes involved in certain known bodily processes such as glucose metabolism or protein synthesis, for example. The emitted positrons travel a very short distance before they encounter an electron, at which point an annihilation event occurs whereby the electron and positron are annihilated and converted into two gamma rays. Each of the gamma ray has an energy of 511 keV, and the two gamma rays are directed in nearly opposite directions. The two gamma rays are detected essentially simultaneously by two of the detector crystals (also commonly referred to as “scintillators” or “scintillator crystals”) in the PET scanner, which are arranged in rings around the patient bore. The simultaneous detection of the two gamma rays by the two detector crystals is known as a “coincidence event.” The millions of coincidence events which are detected and recorded during a PET scan are used to determine where the annihilation events occurred and to thereby reconstruct an image of the patient.
Part of the data acquisition and image reconstruction process involves generating a data structure known as a histogram. A histogram includes a large number of cells, where each cell corresponds to a unique pair of detector crystals in the PET scanner. Because a PET scanner typically includes thousands of detector crystals, the histogram typically includes millions of cells. Each cell of the histogram also stores a count value representing the number of coincidence events detected by the pair of detector crystals for that cell during the scan. At the end of the scan the data in the histogram are used to reconstruct the image of the patient. The completed histogram containing all the data from the scan is commonly referred to as a “result histogram.” The term “histogrammer” generally refers to the components of the scanner, e.g., processor and memory, which carry out the function of creating the histogram.
As PET scanner technology advances, e.g., as detector crystals become faster and as PET scanners include greater numbers of detector crystals, the desired data acquisition bandwidth increases. This increase places greater demands on the histogrammer. In general terms, the function of a histogrammer is to segregate and count events of a multi-type event stream, providing individual counts for each unique event type. For each event in the event stream, the histogrammer reads the current count value in a cell of the histogram, modifies the count value by incrementing or decrementing it, and writes the modified value back to the cell. In current PET scanners, the histogrammer may be required to process millions of events per second. Next generation PET scanners will likely place even higher demands on the speed and memory utilization of the histogramming function. The present invention addresses these needs.
Additionally, it is conjectured that high resolution Positron Emission Tomography (PET) Time Of Flight (TOF) information can be used to improve the image quality of images produced from PET acquisitions. Time of Flight refers to the time difference in detection of the two gamma rays that were produced from a given positron annihilation. TOF is relative to the detector ring diameter and the location of the positron annihilation within the scan field of view. PET detector and acquisition electronics timing resolution have progressed such that sub-nanosecond resolution time of flight difference measurement is achievable and argumentatively clinically cost effective.
PET raw data is nominally collected in sinogram/projection based Line Of Responses (LORs) histograms to compress the acquired data and enhance the performance of the image reconstruction process. The conventional non-TOF PET sinogram/projection based raw data can routinely exceed 64 megabytes per acquisition (frame) and hundreds of megabytes per scan in the Dynamic and/or Gated scan modes. TOF PET adds another dimension to the sinograms/projections, and consequently the sinogram raw data size produced in the Dynamic and/or Gated scan modes would scale by the TOF dimension width (minimally anticipated to be in the range of 32-64) and exceed hundreds of gigabytes and require significant increase in physical memory if current deployed techniques were continued for TOF acquisitions. This could add substantial cost for physical memory to the PET acquisition subsystem and result in an order of magnitude increase in reconstruction processing.
Due to the anticipated reconstruction processing hit and costly increase of Random Access Memory (RAM) projected for TOF sinogram/projection based live event stream histogramming, alternative acquisition methods and raw data formats are being considered industry-wide. Unlike the methods and apparatus disclosed herein, the proposals made to date will most likely have the negative affect on reconstruction time and/or increase the time from end of acquisition until images are presented for medical diagnosis.
The methods and apparatus described herein to produce Compressed Time Of Flight Sinograms for a live or unlist PET TOF coincidence event stream can greatly reduce the amount of physical memory required, and furthermore present the raw data to the image reconstruction process in a LOR ordered format that would significantly reduce the processing time from end of acquisition to presentation of the corresponding images.